Maybe servicing intervals became longer because of technological advancements. Better materials, better tools , smaller tolerances. After let's say 1990, cutting costs became an issue and also because of computer simulations less material was used and parts were manufactured to last for 'a certain period' instead of 'as long as possible'. The last decade, when environmental / exhaust regulations became more strict, I can imagine that servicing is needed earlier to make sure the (very thin) oil remains functional etc.